Recently, portable terminal devices such as cellular telephones and PDAs (Personal Digital Assistants) have become increasingly widespread. One factor that has contributed to such rapid widespread adoption of portable terminal devices is a display device incorporated therein as an output display unit or, typically, a liquid crystal display apparatus. This is because the liquid crystal display apparatus can be driven with less power in principle, and is a low power consumption display device.
In a portable terminal device, a battery of a single power supply voltage is used as a main power supply. In a liquid crystal display apparatus, on the other hand, power supply voltages having different voltage values are used depending upon a logic unit and an analog unit in a horizontal driving system for driving a pixel unit having an array of pixels, and still another power supply voltage having a greater absolute value than that of the horizontal driving system is used in a vertical driving system for selectively driving the pixels in units of rows. Therefore, a plurality of power supply voltages having different voltage values are required for driving the liquid crystal display apparatus.
A power supply generating circuit provided for each of the power supply voltages would increase the complexity of the overall structure of the liquid crystal display apparatus and the cost of the apparatus, thus making it difficult to provide a compact and low-cost portable terminal device incorporating the liquid crystal display apparatus. Accordingly, for example, a liquid crystal display apparatus incorporated in a portable terminal device includes a power supply generating circuit for, based on a power supply voltage of a battery, generating a power supply voltage having a different voltage value therefrom, called a DC-DC converter.
A variety of DC-DC converters are known in the art. One type of such DC-DC converters is a charge pump DC-DC converter. Unlike well-known DC-DC converters having an inductor, advantageously, the charge pump DC-DC converter does not require an external inductor, and contributes to the size reduction of the portable terminal device. Some charge pump DC-DC converters having a function of regulating an output potential are also known in the art.
In a liquid crystal display apparatus incorporated in a portable terminal device, the driving voltage or the driving frequency is reduced to achieve low power consumption in order to extend the life of the battery once charged. However, a liquid crystal display apparatus for this application is designed such that, typically, a DC-DC converter having the regulation ability causes a current to flow in voltage-divider resistors for use in voltage comparison at any time other than the time of voltage comparison during a regulation operation, resulting in high power-consumption loss and low efficiency. For applications to portable terminal devices such as cellular telephones and PDAs, in particular, a reduction in power consumption of the liquid crystal display apparatus itself is an important issue in order to further reduce the power consumption of the portable terminal devices.
The present invention has been made in view of the foregoing problems, and it is an object of the present invention to provide a power supply generating circuit capable of low power consumption of the overall apparatus while reducing the power consumption loss, a display apparatus incorporating the power supply generating circuit, and a portable terminal device using the display apparatus as an output display unit.